1. Field of the invention
The invention relates to a pressure gauge, and in particular to a new optical vacuum pressure gauge.
2. Description of Related Art
Generally, an optical technique is rarely used for a vacuum pressure gauge, but currently, a vacuum system has been combined with an optical system in some areas, such as in a semiconductor industry. When gas inside a vacuum cavity is being vented out, the pressure therein is gradually decreased and the refractive index of the gas is also decreased. If the variation of the refractive index of the gas inside the cavity is accurately measured, then the pressure can be estimated. A total-internal-reflection heterodyne interferometry has the advantages of common path structure, simple setup, easy operation and high precision. Since the total-internal-reflection heterodyne interferometry has a common optical path structure, it cannot be affected by the disturbance from the environment. Furthermore, since it is a phase analyzing approach, it cannot be influenced by the variation of the light intensity, either. As can be know from the above, this approach has characteristics of stability and precision. By use of this approach for measuring the variation of the refractive index of the gas inside the cavity, the required pressure (that is vacuum-degree) can be easily obtained.
The invention is used for measuring the phase of an interference signal, but not for measuring the amplitude or intensity thereof, so the precision of the analysis is higher and it cannot be affected by the variation of the light intensity. In general existing optical pressure gauges, such as the one disclosed in "Split-spectrum intensity-based optical fiber sensor for measurement of microdisplacement, strain, and pressure", Appl. Opt. 35, 2595-2601, (1996) by A. Wang, M. S. Miller, A. J. Plante, M. F. Gunther, K. A. Murphy, and R. O. Claus, deformation caused by pressure difference is mainly measured, wherein the deformation causes light deflection or the change of optical path difference. If the light deflection occurs, the intensity measured on a specific position is also varied. Therefore, the pressure difference can be obtained based on the relationship between the deformation and received intensity. However, since the measured intensity can be influenced by the intensity variation of a light source, a compensation method is needed for modification. In addition, due to limitation of deformation and elastic fatigue, the range of measured pressure is also limited. Furthermore, the pressure difference can also be obtained by measuring the variation of interference fringes based on the variation of the optical path difference. Nevertheless, most general interference approaches do not have common path structures. Therefore, coherence length of laser, disturbance from the environment and precision of analysis must be taken into account when those interference approaches are used for measuring the variation of deformation or refractive index, wherein the disturbance from environment is hard to be overcome.